The present invention relates to a metallic nickel powder suitable for conductive pastes, and in particular, relates to a metallic nickel powder having superior sintering properties and dispersion characteristics, which is specifically suitable for conductive pastes for, and internal electrodes for, multilayer ceramic capacitors.
Noble metals such as silver, palladium, platinum, and gold, and base metals such as nickel, cobalt, iron, molybdenum, and tungsten have been used for electrical parts such as conductive pastes, and have long been specifically used in internal electrodes for multilayer ceramic capacitors. Multilayer ceramic capacitors have a construction such that ceramic dielectric layers and metallic layers used for internal electrodes are alternately laminated, and external electrodes, which are connected to the metallic layers, are connected to both ends of the ceramic dielectric layers. Materials having high dielectric constants, such as barium titanate, strontium titanate, and yttrium oxide, are used as primary components for forming the ceramic dielectric layers. Powders of the above noble metals and the base metals are used in the metallic layers for forming the internal electrodes. Recently, less expensive electrical parts are recently required, and therefore research on multilayer ceramic capacitors using the latter base metallic powders has been aggressively pursued, and nickel powder is typical among these metals.
Multilayer ceramic capacitors are generally manufactured by the following method. That is, a dielectric powder, such as barium titanate, is mixed and suspended in an organic binder, and this is then formed into a sheet by using a doctor blade method so as to produce a dielectric green sheet. On the other hand, a metallic powder for internal electrodes is mixed with an organic compound such as an organic solvent, a plasticizer, or an organic binder so as to produce a metallic paste, and the paste is printed on the green sheet by using a screen printing method. This is then subjected to drying, laminating, firmly pressing, and heating to remove the organic component, and is then sintered at about 1300xc2x0 C. or more. Then, external electrodes are burnt to both ends of the ceramic dielectric layers, and a multilayer ceramic capacitor is thereby manufactured.
In manufacturing process for multilayer ceramic capacitors such as the above, the metallic paste is printed on the dielectric green sheet, and after laminating and firmly pressing, the organic component is vaporized and removed by heating. This heating is usually carried out at 250 to 400 xc2x0 C. in the air. Thus, the heating is carried out in an oxidizing atmosphere, and therefore the metallic powder is oxidized, whereby expansion of volume thereof is generated. Furthermore, the powder is further sintered by heating at a higher temperature after the heating to remove the organic component. This sintering is carried out in a reducing atmosphere, such as hydrogen gas atmosphere. Whereby the metallic powder once oxidized is reduced, and contraction of the volume is generated.
Thus, in the manufacturing process for multilayer ceramic capacitors, the volume changes of the metallic powder due to expansion and contraction thereof are generated by the oxidation-reduction reaction. Similarly, the volume of the dielectric body also changes due to the sintering. However the different materials of the dielectric body and the metallic powder are simultaneously sintered, and therefore sintering properties differ from each other due to changes in the volumes by expansion and contraction of these materials during sintering. For this reason, the strain is generated in the metallic paste layer. As a result, the process has problems in that the laminate construction may be broken and crack or peeling called delamination may occurs
Specifically, sintering in a dielectric body comprising barium titanate as a main component initiates at 1000xc2x0 C. or more, typically at a temperature in the range of 1200 to 1300xc2x0 C. However, sintering in a metallic powder for internal electrodes initiates at lower temperature than that temperature, for example, normally at a temperature in the range of 400 to 500xc2x0 C. in the case of metallic nickel powder, and as a result, the volume changes due to extreme contraction, and the portion between the internal electrode and the dielectric sheet is strained. Thus, the difference between the initiation temperatures for sintering results in difference between sintering properties of the internal electrode and the dielectric body, and this is therefore a primary cause of delamination. Moreover, when sintering is suddenly initiated at low temperatures, volume change in the final period of the sintering is large, so that delamination readily occurs. Therefore, in metallic powders used for internal electrodes, it is desirable that the initiation temperature for sintering be as high as possible and that extreme sintering does not occur.
Various methods for solving the problems of delamination have been proposed. For example, Japanese Unexamined Patent Application Publication No. 246001/96 discloses a metallic nickel powder in which the tap density to specific particle size is higher than the limiting value. In this publication, it has been described that if such metallic nickel powder is used, delamination is not easily occurred when a capacitor is produced by sintering the nickel powder and dielectric body dispersed in a paste.
The above conventional methods can yield some improvement in improving sintering properties. However, these methods are not sufficient to effectively prevent delamination. On the other hand, internal electrodes are required to be formed in thin layers and to have low in electrical resistance in accordance with the trends toward miniaturization and large capacity in capacitors, and therefore metallic powders for internal electrodes are required to be super-fine powders, having not only particle sizes of 1 xcexcm or less, but also particle sizes of 0.5 xcexcm or less. When such a powder consisting of super-fine particles is mixed with an organic solvent, the dispersion characteristics of the powder is deteriorated and the metallic particles agglomerate with each other. As a result, thin layers in internal electrodes cannot be easily formed due to an increase in the number of coarse particles, and bumps and depressions formed on a surface of electrodes may cause short circuiting and also may result in delamination. Therefore, further improvements in dispersion characteristics of metallic powders in organic solvents to form conductive pastes are desired.
Furthermore, as mentioned above, multilayer ceramic capacitors with internal electrodes made from base metallic powders, typified by nickel, are researched according to the requirements for inexpensive electrical parts. However, further development of metallic powders which can prevent occurrence of delamination in this state and are suitable for conductive pastes, has been required.
Therefore, an object of the present invention is to provide a metallic nickel powder in which superior sintering properties are exhibited during production processes for multilayer ceramic capacitors, and superior dispersion characteristics are exhibited in forming conductive pastes, thereby preventing occurrence of delamination. More specifically, the present invention provides a metallic nickel powder suitable for conductive pastes, in which the volume changes and the weight changes due to the oxidation-reduction reaction during sintering are small, the initiation temperature for sintering is high compared to that of conventional metallic nickel powders, and is near the sintering initiation temperature of dielectric bodies used in producing multilayer ceramic capacitors, whereby occurrence of delamination can be prevented and dispersion characteristics in solvent are superior.
The inventors have performed intensive research with regard to dispersion characteristics of a metallic nickel powder. As a result, they have reached a reasoning in which the powder clumps by the polarity of a OH group in nickel hydroxide (for example, Ni(OH)2) which is included in the oxide film of the metallic nickel powder, whereby the dispersion characteristics are deteriorated. Therefore, a metallic nickel powder of the present invention was made as a result of various experiments based on the above reasoning, and it is characterized in that the oxygen content is 0.1 to 2.0% by weight and in that there is no absorption peak at wavelengths ranging from 3600 to 3700 cmxe2x88x921 in the infrared spectrum.
In the case in which metallic nickel powder containing oxygen is subjected to infrared spectroscopic analysis, there is a strong peak at about the 3644 cmxe2x88x921 wavelength. This peak is due to an OH group forming a chemical bond with metallic nickel. In the present invention, metallic nickel powder exhibiting no such peak is used, whereby as many OH groups as possible are removed so that clumping of powder particles due to the polarity thereof is suppressed.
The above peak is not due to water, alcohol, etc., physically adsorbed on the surface of the metallic nickel, and is instead due to nickel hydroxide. A metallic nickel powder of the present invention does not exhibit a strong peak at a wavelength of about 3644 cmxe2x88x921 in the infrared spectrum as described above. The xe2x80x9cpeakxe2x80x9d does not include, for example, small noise peaks, etc., and instead refers to a peak of which the strength or area thereof can be measured when a base line is drawn. A metallic nickel powder of the present invention includes oxygen, and in view of possible application to multilayer ceramic capacitors, the oxygen content is preferably 0.1 to 2.0% by weight, is more preferably 0.3 to 1.0% by weight, and is most preferably 0.3 to 0.8% by weight.
Particle properties of the metallic nickel powder of the present invention are not particularly limited if nothing interferes with use in conductive pastes. However, since miniaturization of multilayer ceramic capacitors is required to reduce the weight of and compactly design recent electronic devices of which they are parts of, it is also required that particle size in metallic powders used for internal electrodes be smaller than ever. Therefore, average particle size of the metallic nickel powder according to the present invention is preferably 0.05 to 1 xcexcm, and a fine particle ranging from 0.1 to 0.5 xcexcm is even more preferable. The specific surface area of the metallic nickel powder by BET is preferably 1 to 20 m2/g. In addition, the particle shape of the metallic nickel powder is preferably sphere in order to improve sintering properties and dispersion characteristics.
In the metallic nickel powder of the present invention, the metallic nickel content measured by an X-ray photoelectron spectroscopy is preferably 0 to 10 atomic percent. The metallic nickel content of 0 atomic percent indicates that the X-rays were blocked by an oxide film and did not reach the metallic nickel, and in this case, the oxidation of the metallic nickel powder is most suppressed. The metallic nickel content is preferably 0 to 8 atomic percent, and is more preferably 0 to 5 atomic percent.
The X-ray photoelectron spectroscopy (hereinafter referred to as XPS) measures the photoelectron spectrum using emission of photoelectrons generated from the inner-shell electron level according to the wavelength of the excited X-ray. This method is primary applied to the analysis of solid samples, and is widely applied to identify elements and for quantitative analysis of elements which are included at a depth from which a photoelectron can escape, that is, from a very thin layer on the surface of a solid, from which an electron can be emitted from the solid without encountering inelastic scattering.
In a case in which the metallic nickel having the oxide film is typically analyzed by using the XPS, the content of the nickel component, metallic nickel, nickel oxide, and atomic nickel due to nickel hydroxide, can be respectively identified, and quantitative analysis thereof can be carried out by comparison with a standard sample. Since the oxide film covers the surface of the metallic nickel powder, of the nickel components identified and quantitatively analyzed by the XPS, the metallic nickel is the metallic nickel under this oxide film. That is, the thinner the oxide film, the greater the ratio of the metallic nickel, and the thicker the oxide film, the smaller the ratio of the metallic nickel. Moreover, since a relationship exists between the accuracy of the oxide film and the ratio of the metallic nickel, even if the metallic nickel powder has oxide films of equal thickness, the lower the accuracy, the greater the ratio of the metallic nickel, and the higher the accuracy, the smaller the ratio of the metallic nickel.
In the above metallic nickel powder, metallic nickel on the surface is at a relatively low concentration of 0 to 10 atomic percent. In other words, the oxide film formed on the surface of metallic nickel powder of the present invention is a uniform oxide film having a definite thickness. Thus, when carrying out heating, in particular, when heating to remove organic components in the temperature range of 300 to 400xc2x0 C. in production processes for multilayer ceramic capacitors, the change in volume and weight due to the oxdation-reduction reaction of nickel can be suppressed somewhat by forming the relatively strong oxide film.
Since the above metallic nickel powder has the strong oxide film on the surface thereof, the initiation temperature for sintering thereof is higher than for conventional metallic nickel powders, and it is nearer to the sintering initiation temperatures for dielectrics used for multilayer ceramic capacitors. Therefore, in the metallic nickel powder of the present invention, oxidation properties and sintering properties in heating are even more superior than in the conventional metallic nickel powder, whereby the dispersion characteristics thereof can be effectively improved.
Furthermore, the thickness of the above oxide film is preferably 2 nm or more, is more preferably 2.5 nm or more, and is most preferably in the range of 2.5 to 5 nm.
The above metallic nickel powder may be produced by well-known methods such as chemical vapor phase methods, liquid-phase methods, plasma methods, etc. In particular, vapor phase reduction methods in which the metallic nickel powder is formed by bringing into contact metallic nickel chloride gas, with a reducing gas, are preferable methods because the particle size of the metallic nickel powder to be formed can be easily controlled, and moreover, spherical particles can be efficiently produced. A vapor phase reduction method is a production technique in which metallic nickel chloride gas is made to react with a reducing gas such as hydrogen, and it can form the metallic nickel chloride gas by heating and vaporizing solid nickel chloride, in a manner similar to conventional methods. However, in view of antioxidation and energy efficiency for the nickel chloride, a production method in which metallic nickel chloride gas is continuously generated by being brought into contact with chlorine gas and metallic nickel, this metallic nickel chloride gas being directly supplied in the reduction process, and nickel chloride being continuously reduced by being brought into contact with a reducing gas, is advantageous.
In production processes for metallic nickel powders using vapor phase reduction reactions, atomic nickel is formed at the instant when the metallic nickel chloride gas comes in contact with the reducing gas, and ultrafine particles form and grow by collision and bonding of the metallic nickel atoms with each other. Particle size in the formed metallic nickel powder is determined by conditions in the reduction process, such as partial pressure and temperature of the metallic nickel chloride gas. According to the above production technique for metallic nickel powder, the metallic nickel chloride gas is generated in an amount corresponding to the supplied amount of chlorine gas, whereby the amount of metallic nickel chloride gas supplied in the reduction process can be controlled by changing the amount of chlorine gas supplied. Furthermore, the present invention differs from methods in which the metallic nickel chloride gas is generated by heating and vaporizing solid nickel chloride, and the metallic nickel chloride gas is generated by a reaction between the chlorine gas and the metallic nickel. The amount of carrier gas used can thereby be decreased, and moreover, depending on the production conditions, may not be necessary. Therefore, it may be possible to reduce costs by decreasing the amount of carrier gas used and decreasing the heating energy therefor.
The partial pressure of the metallic nickel chloride gas can be controlled in the reduction process by mixing inert gas with the metallic nickel chloride gas generated in the chlorination process. Thus, the particle size of the metallic nickel powder can be controlled by controlling the amount of chlorine gas supplied or the partial pressure of the metallic nickel chloride gas supplied in the reduction process; therefore, the particle size of the metallic nickel powder can be made uniform and be optionally set.
The above-described production conditions for the metallic nickel powder by the vapor phase reduction method cannot be absolutely specified; however, the metallic nickel as a starting material is preferably in granules, pellets, or plates having particle sizes of about 5 to 20 mm, and the purity thereof is preferably 99.5% or more. When a production technique is adopted in which the metallic nickel chloride gas is formed by reacting this metallic nickel with chlorine gas, it is necessary that the reaction temperature be 800xc2x0 C. or more, but be less than the melting point of nickel of 1453xc2x0 C., in order to sufficiently drive the reaction. In practice, the reaction temperature is preferably in the range of 900 to 1100xc2x0 C. in view of the reaction rate and durability of the chlorination reactor. Here, the formed metallic nickel chloride gas is subjected to catalytic reaction with a reducing gas, such as hydrogen gas, etc., by direct supply in the reduction process. At this time, an inert gas, such as nitrogen, argon, etc., is mixed with the metallic nickel chloride gas at 1 to 30 mol percent, and this mixed gas may then be introduced into the reduction process. The temperature of the reduction reaction may be above a temperature at which the reaction will sufficiently progresses toward completion; however, the temperature is preferably less than the melting point of nickel since handling is easier if solid metallic nickel powder is formed, and it is in practice in the range of 900 to 1100xc2x0 C. in view of economic efficiency.
Thus, the metallic nickel powder is formed by carrying out a reduction reaction, and the formed metallic nickel powder is then cooled. During cooling, it is desirable that gas flowing at 1000xc2x0 C. near the completion of the reduction reaction is rapidly cooled to about 400 to 800xc2x0 C. by blowing inert gas such as air, nitrogen gas, etc. A metallic nickel powder having desired particle size can thereby be obtained by preventing the generation of secondary particles in which primary particles of the formed nickel agglomerate with each other. Thereafter, the formed metallic nickel powder is separated and recovered by, for example, one or a combination two or more of means including a bag-filter, separation by collecting in water or oil, and magnetic separation.
It is preferable that metallic nickel powder produced in the above manner is subjected to heating in an oxidizing atmosphere. The type of atmosphere for the heating is not limited, so long as it is an oxidizing atmosphere. For example, the heating may be carried out in the air, and it may also be carried out in an oxygen gas atmosphere diluted with argon gas.
The heating in the oxidizing atmosphere is preferably carried out in a temperature range of 200 to 400xc2x0 C., and in particular, in a temperature range of 230 to 300xc2x0 C. When the heating is carried out at temperature exceeding the upper limit of this temperature range, a problem occurs in that the metallic nickel powders agglomerate with each other during sintering. When the heating temperature is lower than the lower limit of the above temperature range, the heating time must be considerably prolonged and is therefore not practical. It is desirable that the time taken for the heating be selected to be in the range of 1 minute to 10 hours, and in particular, 5 minutes to 1 hour. When the upper limit of this time range is exceeded, the metallic nickel powders agglomerate with each other during sintering, whereby particle growth occurs. In contrast, when the heating time is less than the above lower limit, the heating cannot be sufficiently conducted.
A metallic nickel powder obtained by the vapor phase reduction method may be subjected to the above heating immediately. For this reason, if nickel powder is left to stand in the air immediately being produced after, nickel hydroxide is formed by absorbing water, whereby the above heating time is necessary to remove this nickel hydroxide.
According to the metallic nickel powder of the present invention, by carrying out heating of a metallic nickel powder, produced by the above method, in an oxidizing atmosphere, a strong peak due to nickel hydroxide is not detectable by infrared spectroscopy. The initiation temperature for sintering of the metallic nickel powder when heating is carried out tends to be shifted to the higher temperature side, compared with the case in which the same heating is not carried out. As described above, this means that the occurrence of delamination is suppressed during the baking of the multilayer ceramic capacitor.
The metallic nickel powder of the present invention produced in the above manner exhibits superior sintering properties in production processes for multilayer ceramic capacitors, and provides effects in which the occurrence of delamination is suppressed. Specifically, according to the present invention, the initiation temperature for sintering is higher than in the conventional metallic powder and is approximately at the initiation temperature for sintering of a dielectric used in producing multilayer ceramic capacitors, and in addition, it exhibits superior dispersion characteristics in solvent, whereby a metallic nickel powder in which occurrence of delamination is suppressed is obtained.